A fluorescent light tube is a type of low-pressure discharge lamp which in most cases is designed as a closed glass tube (which, for example, can be straight, circular, or bent in a U-shape) with a cathode at each end. The glass tube encloses a gas filling containing mercury vapour. When the fluorescent light tube is switched on, an electron current passes through the gas filling from one cathode to the other. Mercury atoms in this gas filling are then acted upon by the electron current in such a way that they emit invisible ultraviolet radiation. On the inner side of the glass tube there is a layer of fluorescent powder (fluorescent substance) which has the property of converting the short-wave ultraviolet radiation into long-wave visible light.
Since a fluorescent light tube therefore contains a certain (albeit small) quantity of mercury, it is extremely important, from the conventional point of view, when disposing of used fluorescent light tubes, to deal with the mercury from these tubes in an efficient and safe manner. Discarded fluorescent light tubes must therefore be treated as exceptional hazardous waste which is sent to a special destruction plant where the mercury can be dealt with and recovered in a manner which is efficient and, from the point of view of environmental protection, completely satisfactory.
In a fluorescent light tube which is to be disposed of, the mercury which has condensed from the mercury vapour is found on the inner side of the glass tube wall, and since this inner side is coated with a fluorescent powder layer, most of the condensed mercury is therefore found in the fluorescent powder.
Additional mercury, in the form of mercury oxide and/or amalgam, is also found as precipitated particles which have deposited on the inner side of the glass tube wall, especially in the vicinity of the cathodes at the ends of the fluorescent light tube.
Analysis of various parts of crushed or ground fluorescent light tubes has additionally shown that the so-called cathode screen (usually a ferrous strip bent round the cathode) can itself have a relatively high concentration of mercury, for which reason it should be the subject of special treatment for separating off mercury, for example distillation.
A number of methods are already known for dealing with the mercury-containing fluorescent powder from discarded fluorescent light tubes. In one of these known procedures, the cathodes and the end sections including the metal end-sleeves (the end sockets provided with contact pins) of the fluorescent light tube are cut loose or cut off from the rest of the tube so that the latter is open at its ends. The fluorescent powder can then be suctioned or raked out of the tube with the aid of some suitable type of plunger-like scraper tool which has a long shaft and which is pulled or pushed through the glass tube, open at its ends, from one end to the other end.
While it is true that most of the mercury-containing fluorescent powder can in this way be cleared out of the glass tube cut off at its ends, the mercury-containing fluorescent powder and the mercury oxide found in the cut-off end sections of the fluorescent light tube cannot be dealt with and recovered in this way. The cut-off end sections can of course themselves be after-treated and distilled.
When the destruction of discarded fluorescent light tubes involves the fluorescent light tubes being crushed in their entirety (for example in a screw feeder), this also has the consequence that the lead oxide glass which is used as the end seals of the glass tube and as cathode holders will be mixed with the crown glass from the remaining parts of the fluorescent light tubes. This is a considerable disadvantage since it means that the usability of the recovered glass as a raw material for the manufacture of new glass products is thereby limited. The value of the recovered glass as a recovered raw material for the manufacture of new glass products is in this way drastically reduced.
In EP-A-0420367 there is disclosed a method and device for sanitation of mercury-containing lamps. DE-A-4030732 discloses a method for recycling of fluorescent lamps/tubes and broken pieces thereof. WO-A-9301889 discloses a method for removing metal electrical tips from fractured glass derived from a fluorescent tube crusher, and a tip separator for use with crushed fluorescent light tubes.